This invention relates to an image processing method and apparatus and to a zoom processing method used when bi-level image data is zoomed and output to a recording device having a high pixel density, particularly a zoom processing method used when bi-level image data, which includes a pseudo-halftone image employed in a facsimile machine or electronic file, is subjected to a resolution conversion and then output to a recording device.
Many methods such as the SPC method, 9 segmentation method and projection method have been studied and announced as methods of zooming bi-level images. First, the SPC method performs zooming by simply duplicating or subsampling pixels or lines. Since this method has a simple hardware configuration, it is employed in comparatively inexpensive facsimile machines. The projection method is accompanied by arithmetic processing, unlike the SPC method, and involves projecting a zoomed image on an original image and calculating the density value of zoomed image data from the density of the original image and the percentage of the area which the original image occupies in a zoomed pixel subdivision. This method provides comparatively good results with regard to a bi-level image such as a character or line drawing.
A disadvantage of these methods is that a pseudo-halftone image obtained by error diffusion method or the like develops a moire pattern and undergoes a conspicuous deterioration in image quality.
FIG. 12 illustrates the result of enlarging image data, which consists of eight pixels in the main-scan direction and eight pixels in the sub-scan direction, by a factor of 5/4 by the SPC method. As shown in FIG. 12, the fourth and eighth rows in the sub-scan direction are duplicated by processing and so are the fourth and eighth columns in the main-scan direction. The SPC method is such that regardless of the fact that the ratio of black to white pixels in the original image is 1:1, the ratio of black to white pixels in the image data produced by processing is 46:54, meaning that the density of the original image has not been preserved. It will be understood that the expression of halftones is poor.
A combination of the projection method and error diffusion method and a combination of a binary-multivalue conversion by filtering and error diffusion method have been proposed as methods of obtaining comparatively good results when applied to pseudo-halftone images. With these techniques, however, considerable hardware resources such as multipliers and line buffers, the latter of which are for error diffusion method, are required. This results in a more costly apparatus.